The present invention relates to optical communications devices and, more particularly, to optical couplers.
An optical coupler is a device for exchanging light between two optical waveguides. An optical waveguide is a device for transmitting light over long distances with low losses. It consists of a linearly extended guide portion, having a relatively high index of refraction, encased in a cladding having a lower index of refraction. Light is confined to the guide portion by total internal reflection. Common examples of optical waveguides include planar waveguide structures, which, for the transmission of infrared light, often are made from semiconductors in the same way as integrated circuits, and optical fibers. In an optical fiber, the guide portion conventionally is called a "core". The illustrative examples herein include both systems based on optical fibers, as embodiments of the present invention, and systems based on planar waveguide structures, as computational model systems for the theory of the present invention. It will be understood that the principles of the present invention apply to all optical waveguides, and not just to planar waveguide structures and optical fibers.
A directional coupler, in particular, consists of two parallel waveguides in close proximity to each other. The theory of directional couplers is described in D. Marcuse, Theory of Dielectric Optical Waveguides, Academic Press, Second Edition, 1991, Chapter 6, which is incorporated by reference for all purposes as if fully set forth herein. Two identical waveguides, far apart from each other, have identical propagation modes, with identical propagation constants. As the two waveguides are brought closer to each other, pairs of corresponding modes become coupled. The solutions of Maxwell's equations are, to a close approximation, sums (even symmetry) and differences (odd symmetry) of the corresponding uncoupled modes, each solution having its own propagation constant that is slightly different from the propagation constants of the corresponding uncoupled modes. Monochromatic light entering a directional coupler via the guide portion of one of the waveguides in one uncoupled mode thus is a linear combination of two coupled modes. Therefore, this light is exchanged between the guide portions of the two waveguides. After propagating through the directional coupler for a distance called the "beat length", the light has been transferred entirely to the guide portion of the other waveguide. Of course, if the directional coupler is longer than the beat length, the light returns to the guide portion of the first waveguide. The beat length is inversely proportional to the difference between the coupled propagation constants. Specifically, the beat length L=.pi./(.beta..sub.e -.beta..sub.o), where .beta..sub.e is the propagation constant of the coupled even mode and .beta..sub.o is the propagation constant of the coupled odd mode. These propagation constants are functions of the indices of refraction of the guide portions and of the intervening optical medium, and of the wavelength of the light.
The closer the guide portions are to each other, the larger the difference between the coupled propagation constants. In practical optical couplers of this type, in order to keep the beat length, and hence the length of the device, on the order of centimeters, the distance between the coupled guide portions often must be on the order of micrometers. This dimensional restriction increases the cost and complexity of the couplers. There is thus a widely recognized need for, and it would be highly advantageous to have, an optical coupler including a mechanism for reducing the beat length, or, equivalently, allowing the coupled guide portions to be spaced farther apart for a given beat length. If such a mechanism were reversible, the resulting optical coupler also could be used as an optical switch or a variable coupler.